Before the telephone companies relinquished complete control over the whole telephone system, including the telephone lines and the telephone equipment connected to the lines, few significant problems were experienced by telephone subscribers on telephone party lines. The telephone sets supplied by the telephone company were specifically adapted or "wired" to respond only to a single one of the four possible ringing signals which the telephone company might apply to the telephone party line to ring or address a specific subscriber. The four different ringing signals are a positive ringing signal on the tip conductor, a negative ringing signal on the tip conductor, a positive ringing signal on the ring conductor and a negative ringing signal on the ring conductor. So long as the telephone company was able to specifically adapt each telephone set to respond to only one of these ringing signals, reliable ringing operation was possible.
Significant ringing problems on party lines began arising when different types of telephone sets and equipment became available from manufacturers and sources other than the telephone company. Most of the commercially available telephone equipment responds to any type of ringing signal because the equipment is manufactured to be used on private telephone lines and not party lines. Such equipment cannot readily be used on party lines without modification, and consumers are unable, or do not have sufficient information, to make the appropriate modifications.
To attempt to solve the problem of adapting varieties of different telephone equipment to party line operation, selective ringing circuits have been devised. The typical selective ringing circuit is a separate piece of equipment which is electrically connected between the party line and the telephone equipment, either in a telephone company central office or at the point of use of the telephone equipment. The typical selective ringing circuit is subject to certain disadvantages. One significant disadvantage is that a different selective ringing circuit is required to obtain the desired response from each of the four party line ringing signals. A substantial inventory of each of the four different types of selective ringing circuits must be kept available. Furthermore, many of the selective ringing circuits are not as effective or precise in operation as is desired. Problems have been encountered in responding to the correct or selected ringing signal and in rejecting the other undesired ringing signals. Some of these problems occur as a result of peculiarities in the telephone systems and lines from one geographical area to the other. Many of the currently available selective ringing circuits will not respond properly to different strengths and levels of signals, to switching signals, and to other peculiarities, all of which pose problems of reliability in operation. Other currently available selective ringing circuits modify the normal telephone signals to the point where the signals are incapable of producing an adequate response in the telephone equipment, even if the selective ringing circuit achieves adequate signal recognition and rejection.
In two party-party line operation, it is common practice for the telephone company to identify the second party for long distance billing purposes by placing a predetermined ground resistance, nominally 2600 ohms, on the line when the second party places a long distance call. The second party is identified by the added resistance effects on the line. Commonly available telephone equipment does not include the capability for selectively switching the predetermined ground resistance into the line for automatic party or number identification purposes.
Most modern telephone equipment, including that supplied by the telephone company, lacks the capability to achieve communication with computers and comparable devices, without modification. Most communication with computers is achieved through tone-generating key pads which generate AC signals representative of the keys depressed. Such key pads require a constant DC voltage to operate. Since it is typical practice for the telephone company to reverse the polarity of the telephone line of a calling party when the called party answers, this has the effect of disabling the key pad of the calling party and rendering it useless for communication under such circumstances. With special modifications to the telephone set, the problem of a disabled key pad can be overcome, but again, such modifications are not within the capability of a consumer of conventional telephone equipment.